Alias variable subscripts cause compilation errors

[Martin Sjölund]

x3 cannot be found because it is not replaced by the backend :)

class A
  Integer x1 = if time < 0.5 then 1 else 3;
  Integer x3 = x1;
  Real x2;
  Real r[1] = {2.5};
algorithm
  x2 := r[x3];
end A;